A number of ciliates which live in water or soil may contaminate feces and develop coprophilically. They are common in old feces, especially if it has been in contact with the ground, but may also appear in feces taken directly from an animal. Cysts ingested by livestock in feeding or drinking may pass thru the intestinal tract unharmed, and trophozoites may emerge and develop as the feces stands. Horse and ruminant feces which have been cultured for nematode larvae often contain large numbers of small ciliates. Some of these are probably Colpidium, Chilodonella and Cyclidium.
Nyctotherus faba Schaudinn, 1899 has been found in human feces on occasion (Wichterman, 1938). It belongs to the heterotrichorid family Plagiotomidae. Its body is reniform, covered with cilia, and 26 to 28 u long. The peristome begins at the anterior end, turns slightly to the right and ends in the cytostome at the middle of the body. The cytopharynx is a long tube and contains an undulating membrane. The macronucleus is about the middle of the body. It is spherical, and its chromatin is arranged in 4 or 5 large, solid bodies on the nuclear membrane, while the remainder of the nucleus is chromatin-free.
Noble (1958) found that a Nyctotherus-like ciliate about 15 to 30 u long appeared in fecal samples from Wyoming sheep and elk after storage at 4° C for about 30 days. A smaller ciliate about 10 to 12 u long also appeared in the elk feces at about the same time. The smaller ciliates persisted for a few weeks and the Nyctotherus-like ones for about twice as long.
Balantiophorus minutus Schewiakoff, 1893 (syn., Balantidium minutum Schaudinn) occurs occasionally in contaminated human feces (Watson, 1940, 1945, 1945a). It beongs to the holotrichorid family Pleuronemidae. It is ovoid, with the narrow end anterior and with the anterior end bent ventrad, giving the ventral surface a hollowed appearance. It measures 12 to 54 by 7 to 33 u, but is usually 25 to 45 u long. The peristome is in the middle of the anterior half of the body. The adoral zone of membranelles on its left, posterior and right borders forms a sac-like structure which is conspicuous when expanded but which can be retracted into the peristome and become invisible. The cytopharynx is funnel-shaped. The body is uniformly covered by 12 rows of setiform cilia, of which only 6 extend anterior to the peristome. The macronucleus is central and ellipsoidal. There is a posterior contractile vacuole.
The taxonomy and bionomics of these and other coprophilic protozoa have been reviewed by Alexeieff (1929) and Watson (1946). The latter listed 51 species of flagellates, 18 of amoebae and 18 of ciliates which have been found in feces, but many of these need further study.